Polymer electrolyte membrane fuel cells (PEMFC) are expected to provide higher efficiencies, fewer environmental pollutants, and reduced operating and maintenance costs than traditional power sources. An important component of a PEMFC is a polymer electrolyte membrane (PEM). The range of potential candidates for use as membrane materials in PEMFCs is limited by a number of requirements, including chemical and mechanical stability, high ionic conductivity, and low reactant permeability. Developments have been made in the use of sulfonic acid functionalized polymers, including membranes such as Nafion® perfluorosulfonic acid membranes.
Known membranes made from sulfonic acid functionalized polymers have been found to have inadequate performance at temperatures greater than 100° C. due, in part, to the dependence of the membranes on water for proton conduction. Above 100° C., pressure constraints limit the amount of water that can be used to hydrate a membrane. At relatively low levels of hydration, insufficient water is present within the membrane to transport protons. In addition to improved performance at higher temperatures, it is also desirable to have improved mechanical stability at such temperatures and decreased methanol permeability in membranes used in direct methanol fuel cells.
Alternatives to perfluorosulfonic acid membranes include membranes based on aromatic engineering polymers. Poly(arylene ether)s and poly(arylene ether sulfone)s are engineering polymers known for their chemical and thermal stability. Poly(arylene ether)s and poly(arylene ether sulfone)s can be sulfonated to produce sulfonic-acid functionalized aromatic polymers as disclosed, for example, by A. Noshay and L. M. Robeson in “Sulfonated Polysulfone”, J. Appl. Polym. Sci. 20, p. 1885 (1976). However, due to relatively poor control inherent in the process, post-polymerization sulfonation can result in sulfonation on the most electron-rich aromatic rings, which are also the most activated to subsequent decomposition of the sulfonic acid. Additionally, only one sulfonic group per repeat unit is typically achieved.
Another method for producing sulfonic-acid functionalized aromatic polymers is by polymerizing sulfonated monomers, as disclosed, for example by F. Wang et. al, “Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes”, Journal of Membrane Science 197 (1–2), pp. 231–242 (2002). However, the proton conductivity of sulfonated aromatic polymers made by either post-sulfonation or polymerizing sulfonated monomers by either of the two methods discussed hereinabove is limited by the acid strength of the aromatic sulfonic acids, especially at low relative humidity.
A need remains for conductive membranes suitable for use in applications such as fuel cells. A need also remains for engineering polymers having the advantageous thermal properties found in poly(ether sulfones). For use as conductive membranes, it is also desirable that such polymers have adequate mechanical strength, improved conductivity at higher temperatures, e.g., above about 80° C. For some applications it is desired that the conductivity be higher than about 50 milliSiemans per centimeter (mS/cm.).